The present invention relates to the field of wear resistant castings and their manufacture. More specifically, the present invention relates to the field of wear resistant earthworking castings and penetration resistant security devices.
In the field of earthworking equipment, the useful lifetime of the teeth contacting the formation being worked is important to the economic success of the work being performed.
The lifetime of these teeth are affected by the environment in which they operate. Typically, the environments encountered may produce conditions of abrasive wear, impact loading, temperature variation, vibration and corrosion at the teeth surface, all factors which tend to reduce the lifetime of the tooth or tool. The high cost in terms of downtime and tool cost for the replacing of worn out and broken tools has led to the development of a wide variety of tools designed to provide improvements in their in-service lifetimes.
In some cases, these improved tool designs have included the embedding of carbide into the tool working surface through casting processes (see, for example, U.S. Pat. Nos. 4,024,902 and 4,140,170).
These casting techniques present problems when it is desired to produce castings having relatively thin cross sections or when it is desired to place carbide particles on the surface of a vertically extending appendage, as well as a horizontal portion, of a casting.
In order to minimize dissolution of the carbide particles during casting, and the resulting brittle eta phase (M.sub.6 C or M.sub.12 C carbide containing tungsten and iron) produced at the carbide-steel interfaces, the cemented carbide particles utilized typically should have a size of at least 1/8 inch. Increasing the size of the particles reduces the carbide-steel interface area. However, in thin sections of a casting having a thickness only slightly larger than the carbide size, the carbides can act in conjunction with the mold to rapidly and excessively chill the molten metal flowing between the carbides and thereby cause incomplete filling in these thin sections.
It is also impractical to hold large cemented carbide particles uniformly dispersed along a vertical section of a casting without filling that section with carbide from the bottom up so as to hold the carbides in position during casting. This can lead to the aforementioned voids and/or incomplete filling due to excessive chilling of the melt.
Australian Patent No. AU-B1-31362/77 attempts to avoid the aforementioned casting problems by milling a heat treatable low alloy steel powder together with a tungsten carbide powder or tungsten molybdenum solid solution carbide powder, and then pressing and sintering to substantially full density a compact of the resulting mixture. Low alloy steel is then cast around the sintered steel-carbide compact to form a finished component. This Australian patent, however, limits the steel powders used to low chromium content steel.